Regulation of mouse thymidylate synthase gene expression in growth-stimulated cells: upstream S phase control elements are indistinguishable from the essential promoter elements.

Expression of the mammalian thymidylate synthase (TS) gene in growth-stimulated cells is closely coordinated with entry into S phase. Previous studies with transfected TS minigenes have shown that sequences upstream of the coding region as well as an intron in the transcribed region are both necessary for proper regulation of TS mRNA content in growth-stimulated cells. The goal of the present study was to identify the upstream regulatory elements. Minigenes consisting of TS 5' flanking sequences linked to the TS coding region (interrupted by introns 1 and 2) were stably transfected into mouse 3T6 cells. Deletion and site-directed mutagenesis of the 5' flanking region revealed that there is a close correspondence between the upstream sequences that are necessary for S phase regulation and the 30 nucleotide region that is essential for promoter activity. These observations raised the possibility that regulation of the TS gene occurs at the transcriptional level. However, nuclear run-on assays showed that the rate of transcription of the TS gene changed very little during the G1-S phase transition. Furthermore, when the TS promoter was linked to an intron-less luciferase indicator gene, there was no change in expression following growth-stimulation. Therefore it appears that the TS gene is controlled primarily at the posttranscriptional level, and that the TS essential promoter region is necessary (although not sufficient) for proper S phase regulation.